1. Field of the Invention
The present invention relates to a method and apparatus for processing signals in a code division multiple access (CDMA) radio communication system.
2. Background of the Related Art
In order to implement an adaptive array antenna for performing spatial and temporal processes, a CDMA system typically uses a frequency down converter for converting a very high frequency signal into a baseband signal, an analog-to-digital (A/D) converter for converting an analog signal into a digital signal, a beamformer for performing the spatial process, a rake receiver for performing despreading and the temporal process, and a digital demodulator for demodulating a digitally modulated signal.
FIG. 1 shows the construction of a conventional rake receiver of a CDMA system for performing the temporal process only. In the frequency selective fading environment, the degree of delay of multiple paths becomes larger than a symbol period. In the CDMA system, the signal process is performed in the unit of a chip, and based on the assumption that one symbol corresponds to one chip, an intersymbol interference (ISI) that is larger than one chip period can be discriminated through the rake receiver.
As shown in FIG. 1, in case that multiple paths exist, the L multiple paths having time differences can be discriminated by using L fingers. Each finger includes a time delay 101, a correlator 102, and an integrator 103. More specifically, in order to discriminate the respective paths, the rake receiver is composed of several correlators that match the respective time delays of the multiple paths, and their outputs are summed together by the rake receiver to maximize the signal_to_noise ratio (SNR). Generally, components of the multiple paths have different time delays and different incident angles.
A receiver using an existing single antenna can analyze signals on a time base only, and thus only the temporal process is performed using the rake receiver. However, in a smart base transceiver system which has various antennas installed therein, signals on a space base can also be analyzed.
FIG. 2 shows a conventional smart base transceiver system in the form of a two-dimensional rake receiver. In this receiver, signals of respective paths received through the antennas are converted into down frequencies by frequency down converters 201 (step 201), and their correlations are obtained through correlators 202.
In order to use the respective multiple paths having different incident angles, the two-dimensional rake receiver allocates a beamformer 204 to each path, performs the spatial process using weight vectors calculated by a beamforming weight calculation section 203, and combines the processed signals through a rake combiner 206. Then, the two-dimensional rake receiver processes the combined signal using the existing rake receiver, and then combines the signals of the respective paths whose spatial and temporal processes are completed.
In the conventional CDMA system, it is therefore clear that a finger only takes charge of the temporal process in a manner that it is allocated with a path having a different time delay using a signal received through a single antenna. On the other hand, the two-dimensional rake receiver takes charge of the spatial process in such a manner that a finger generates weight vectors based on signals received through a plurality of antennas, combines the respective antenna signals, and then takes charge of the temporal process. Of course, the two-dimensional rake receiver is allocated with signals having the different time delays through the respective fingers, and then combines all the signals whose processes have been completed in the same manner as the conventional system.
However, there is a great difference between performing the temporal process only and simultaneously performing the temporal process and the spatial process.
A beamforming algorithm (i.e., performed by the beamforming weight vector calculation section 203) in the two-dimensional rake receiver refers to obtaining an eigenvector that corresponds to the maximum eigenvalue of an autocorrelation matrix using the autocorrelation matrix composed of signal vectors. At this time, in order to obtain weight vectors to be applied to the beamformer for an IS-95 CDMA system, the conventional receiver performs the beamforming algorithm using both a high-speed signal before being despread to a predefined code and a low-speed signal after being despread to the predefined code.
In addition, in order to apply the adaptive algorithm, the conventional receiver calculates the autocorrelation matrix of the signal obtained by sampling the signal before being despread and the autocorrelation matrix of the signal obtained by sampling the signal after being despread, and then calculates the weight vectors for the spatial process using the respective matrices.
As described above, application of the two-dimensional rake receiver used in the CDMA system to a wideband code division multiple access (WCDMA) system is now under consideration. However, the principle of the rake receiver cannot be applied to the WCDMA system as it is, but its signal processing method should be changed to match the WCDMA system even though the rake receiver can be used in the WCDMA system.
The above references are incorporated by reference herein where appropriate for appropriate teachings of additional or alternative details, features and/or technical background.